Conventionally, some suggestions have been made regarding image signal correction devices. Such a device detects movement of an image and processes an image signal differently between the area with a still picture and the area with a motion picture. For example, Japanese Patent Unexamined Publication No. 2001-34229 discloses a method of correcting dynamic pseudo contour. FIG. 8 is a block diagram of image correction device 20 for correcting dynamic false contour, which is introduced in the aforementioned disclosure. In FIG. 8, image correcting means 170 contains adder 3; still-picture coding circuit 4; motion-picture coding circuit 5; selector 7; differential circuit 11; coefficient circuit group 12; and delay circuit group 13. With this structure, image correcting means 170 provides an image signal with error diffusion. Image correcting means 170 is controlled by the output of movement detecting means 120 such that an error diffusion using the output from the still-picture coding circuit 4 takes place in a still-picture area, while, in a motion-picture area, an error diffusion using the output from motion-picture coding circuit 5 takes place.
In such a correction device, however, upon switching of correction process of image signals according to movement of images, some images have often produced noise with sharp edge (hereinafter referred to as switch shock) at the boundary between the still-picture area and the motion-picture area. To address the problem above, Japanese Patent Unexamined Publication No. 2001-34229 also introduces another image correction device that performs the diffusion process with the use of random numbers so as not to give sharp lines to the boundary area, whereby the switch shock is eased. The conventional image correction device, however, simply provides the boundary area with error diffusion. Therefore, with some images, the switch shock persists due to an insufficient diffusion (or other noise like jitters which is a side effect of the diffusion), and appears along the outlines of the image. Avoiding the inconveniences above has been the obstacle to an intended correction of dynamic pseudo contour.
The present invention addresses the problems above. It is therefore the object of the invention to provide a method and device for image correction capable of not only performing image signal process according to the movement of images, but also suppressing the switch shock.